Switching power converters typically convert a DC voltage into an AC voltage by operating switching elements and then reconvert the AC voltage back to a DC voltage with a rectifier and smoothing circuit. A control circuit may be used to control a duty cycle of the switching elements. Switching power converters allow for a variable output voltage by varying the duty cycle of the switching elements. The ratio of the output voltage and the input voltage is typically determined by the duty cycle of the switching elements.
Synchronous rectification has been developed to reduce rectification losses improving the efficiency of the rectifier. Synchronous rectification proves particularly useful in power converters requiring a low output voltage and a high output current. Synchronous rectification utilizes power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) to rectify the output voltage of a power transformer. The MOSFETs are synchronized to the duty cycle of the switching elements and perform more efficiently than diodes due to a MOSFET's low resistance path during conduction. There are several known techniques to control synchronous rectifiers in a manner to provide a very low resistance path during forward conduction and function as a diode during the blocking period.
The bidirectional current flowing capability of the MOSFETs may allow reverse current to flow into the power converter. Reverse current may occur when a voltage exceeding the output voltage of the power converter is imposed on the output of the converter. The excess voltage charges a capacitor typically at the output of the power converter. When the output voltage of the converter settles, the capacitor discharges. If the synchronous rectifiers are conducting, the current discharged from the capacitor may flow in a reverse direction into the power converter.
In a stand alone power converter, reverse current can occur during step unloading or during start-up. Step unloading occurs when a load that is driven by the converter is removed and may create a voltage overshoot on the output. The voltage overshoot may charge the output capacitor during a no-load condition and raise the output voltage without having a load to discharge it. The control loop circuit may reduce the duty cycle to compensate for the overshoot. Because the control loop is rather deliberate, reverse current may occur while the duty cycle is reduced. During start-up of a stand alone power converter, the control loop circuit gradually raises the output voltage of the power converter to a soft-start reference level. If the power converter is driving a light load, the power converter may miss some pulses from the control loop causing the rectifiers to conduct longer and increasing the risk of reverse current.
For two or more power converters connected in parallel and having OR'ing MOSFETs substitute for OR'ing diodes, the OR'ing MOSFETs act as a short circuit when turned on so that they appear as directly parallel power converters. In a first situation involving parallel power converters, the output voltages from each converter may be slightly different, creating a possible reverse current condition. In a second situation involving parallel power converters, a first power converter may be on while a second power converter is inserted in parallel, such as in a redundant system. If the first power converter has a slightly higher output voltage, it may drive the entire load driving the output of the second power converter toward zero. The control loop circuit may not react quickly enough to increase the output voltage of the second power converter to that of the first power converter, and the second power converter may start to sink the current as its output capacitor is charged to the voltage level of the first power converter. Even when the converter is switching, progressive reverse current may build the output inductor since the synchronous rectifiers are switching.
This situation leads to the reverse operation of the converter transferring energy from the secondary side to the primary side and into the bulk capacitor. Both synchronous rectifiers are turned on during the dead time and just before the start of the next active period one synchronous rectifier is turned off. The reverse current may flow from the output capacitor in to the synchronous rectifier through the output inductor during the dead time. At the end of the dead time, one of the synchronous rectifiers is turned off and the reverse current in the inductor is interrupted swinging the input end of the inductor positive. This may occur after the dead time or it can happen when the switching resumes if pulses are missed. This operation is similar to a boost converter but the direction is reversed and the output voltage is boosted and transferred to the primary side through the power transformer. This causes the bulk voltage to rise, which can ultimately lead to power converter failure. In addition, overvoltage protection circuits typically used for bulk overprotection are ineffective due to reverse conversion from the secondary side.
When the voltage applied to the output of the converter is greater than the output voltage, reverse current starts to build during the dead time. More specifically, reverse current may flow from the output of the power converter through the secondary windings and the synchronous rectifiers to ground. The reverse current through the secondary windings may not be equivalent, however no net flux is induced in the transformer due to the equal and opposite voltages across the secondary windings. At the end of the dead time, one of the synchronous rectifiers is turned off and thus a portion of the reverse current flowing through the output inductor is interrupted. As a result, the output voltage of the power converter swings up to boost operation. The resulting boosted voltage may be clamped to an input bulk capacitor potentially causing damage to the input bulk capacitor, which is undesirable.
It is desirable to prevent reverse current to improve the robustness of power converter design. Various techniques have been proposed to achieve this goal. However, most techniques either simply reduce the reverse current or prevent it by turning off the synchronous rectifiers at a light load. Turning synchronous rectifiers off at light loads may result in an additional power loss at lighter loads and compromises the advantage of the using synchronous rectification.
Other techniques for addressing reverse current utilize a circuit to sense a voltage drop across the synchronous rectifier. The drop across the synchronous rectifier is negative when the current through the switch is in the forward direction, zero when there is no current, and positive when it is in the reverse direction. The synchronous rectifier is turned off when the drop is near zero by comparing the drop against a fixed reference. Since the synchronous rectifier is off when the current becomes zero, reverse current can be prevented. However, in high current low voltage converters, using low resistance MOSFETs for synchronous rectification makes it difficult to sense the voltage drop, and the body diode conduction time cannot be controlled precisely. In addition, if the synchronous rectifier is turned off earlier than required, due to the difficulty in sensing the voltage drop, the excessive body diode conduction will result in a lower efficiency.
Another technique for addressing reverse current utilizes a circuit that turns off the synchronous rectifier when the forward current goes below a certain minimum level, for example 10% of the full load. Since the synchronous rectifier is already in an off condition before the current reaches zero, the reverse current can be prevented. However, at loads when the synchronous rectifiers are off, the losses will be higher resulting in a less efficient operation at light loads. In addition, the output inductor may become discontinuous at light loads degrading regulation and ripple.
Yet another technique for addressing reverse current utilizes a circuit that detects the presence of reverse current and suppresses the reverse current by controlling the duty cycle ratio of the control loop circuit. The reverse current is identified when a voltage across the drain-source terminals of the MOSFET remains near zero in spite of having received an OFF command at its gate terminal. A transistor operates under this condition controlling a second transistor in a manner that increases the duty ratio to suppress excessive build up of reverse current. However, the reverse current is only reduced, not prevented.